This invention relates to a charge composition for smelting furnaces, and a method of forming non-fired agglomerates such as pellets or briquettes therefrom which show excellent reducibility and do not interfere with the proper smelting reactions and temperatures.
In order to facilitate the smelting of a charge including finely divided iron ore while preventing the iron ore fines from being scattered about in the smelting furnace and also insuring satisfactory smelting reactions, it has been the prior art practice to form the charge composition into agglomerates such as pellets or briquettes prior to charging to the smelting furnace.
The utilization of pellets or briquettes in a smelting furnace such as a blast furnace is itself not without problems for the reason that the pellets formed from the charge composition are frequently found to possess an inadequate crushing strength for convenient handling from a travelling grate system to a furnace smelting system as well as for the smooth smelting procedure in which the charged pellets are forced to move downward under the action of gravity and the weight of the pellets accomulated thereon, thereby an excessive amount of fines tend to be formed in the smelting furnace.
One approach to the impartment of an adequate crushing strength to the pellets or briquettes is that the finely divided iron ore is mixed with limestone, coal and bentonite as a binder, and the mixture after, if necessary, moistened with a proper amount of water is agglomerated and then fired at elevated temperatures as high as several hundreds of centigrades to produce indurated pellets or briquettes. This method, however, has a drawback resulting from the provision of the firing step which makes complicated the process for producing iron from the iron ore and consequently increases the production cost of iron. Another drawback is that the gas discharged from the firing step contains poisonous compounds such as SO.sub.x and NO.sub.x which must be removed, or otherwise they would pollute the environment of air.
Many attempts have been made to eliminate the firing step from the processing operation for the production of indurated pellets or briquettes and a technique has been proposed to provide non-fired pellets or briquettes having a crushing strength large enough to prevent an unacceptably excess amount of fines from being produced from the pellets or briquettes during the travelling and smelting operations. For example, the finely divided ore is blended with a binder having a hydraulic property such as Portland cement clinker, then the blend after moistened is formed into discrete moist pellets, and then the pellets are cured as buried in a power matrix which may be identical in material to the used ore. This agglomerate composition of nonfired pellets and the process for making same, however, have much to be desired, which is explained in more detail below.
The prior art non-fired pellets, though having a crushing strength not inferior to that of the fired pellet at ordinary temperature, are found to have inferior reducibility and interfere with the proper smelting reactions and temperatures, because of the scabbing and premature softening phenomena of the pellets occurring as the temperature of the smelting zone ranges from 500.degree. to 1,400.degree. C. The prior art process for making non-fired pellets, on the other hand, as, for example, disclosed in Japanese Patent Publication No. Sho 46-32324, includes a curing step in which the as-formed discrete moist pellets are burried in ore powders as a matrix to avoid the adhesion of the pellets with one another until the cementitious material is completely cured. After the curing has been completed, it is necessary to rid the pellets of the ore powder by means of screening. However carefully the screening is carried out, it is difficult to reduce to zero the residual amount of ore fines adhering to the surfaces of the pellets which when charged into a smelting furnace such as a blast furnace tends to reject some of the adhered ore fines. When the rejected ore fines are accumulated in the furnace, it is found that the degree of passability of forced air or oxygen draft is gradually decreased, and that when the amount of fines produced is excessive, the handling difficulties becomes serious. Furthermore, the screening operation is very time-consuming, and the necessary iron ore powder as the matrix amounts to more than 30% based on the total weight of the charge pellets.
Accordingly, the present invention has for the general object to eliminate the above mentioned conventional drawbacks, and to provide non-fired agglomerates which do not interfere with the proper smelting reactions, and a process for making non-fired agglomerates without the necessity of using the powder matrix which would be otherwise necessary to prevent the adhesion of the pellets during the curing operation.
To achieve this, at first, the composition of agglomerates such as pellets and briquettes is formulated so that the ratio CaO/SiO.sub.2 is in a range of from 1.2 to 1.9, and the amount of slag forming material is in a range of 13% to 19% based on the total weight of the non-fired agglomerate, whereby the drafting aspect of the indurated pellets or briquettes in the smelting zone are improved.
Secondary, the present invention has an object to improve the properties of the agglomerates by adjusting the composition, namely the CaO/SiO.sub.2 ratio and the proportion of the slag forming material through blending of iron ores.
Thirdly, the curing of the as-formed discrete moist agglomerate such as pellets or briquettes is carried out without using any powder matrix to impart excellent high-temperature properties to the resultant cured agglomerates without sacrificing the reducibility of the agglomerate charge, whereby the handling difficulties of the pellets or briquettes due to the adhesion of fines are minimized.
For the purpose of furthermore reducing the possibility of assuming the adhering aspects of the moist pellets during the curing operation with no matrix, an over-coating of an inorganic substrance is applied around each of the moist pellets before subjecting the curing operation.
Fourthly, the present invention has an object to provide an improved agglomeration method which can give satisfactory agglomerates without using any powder matrix during the curing.
These and other features of the present invention will be more readily understood when the following description is read in connection with the accompanying drawings.